Unify workspace panel drag, snap, and motion interactions

[jhe009809-hue]

Summary

• Unifies dragging, collapsing, expanding, and animations across the sidebar, conversation area, and operations panel.
• Crossing a drag threshold immediately triggers collapse or expansion; reversing the drag restores the split.
• Panels resize smoothly while adjacent areas fill the space. Buttons use the same animation behavior.
• The sidebar stays within its width limits. Conversation and operations panels support expanded views while retaining hidden content.
• Keyboard controls, canceled gestures, reduced motion, and narrow-window layouts are supported.

Why

Issue

• Closes #

Scope

Out of scope

Testing

Ran

• ...

Result

• pass/fail:
• if fail, exact files/errors:

CI status

• pass:
• code-related failures:
• external/env/auth blockers:

Manual verification

Evidence

• video/screenshot link, or N/A (docs-only)

Contributor statement

• I have the right to submit this contribution under CONTRIBUTING.md.
• I preserved third-party copyright and license notices, or this change does not add third-party material.

Risk

Rollback